Merge pull request #4493 from ARMmbed/fix_timer_stack

Fix typo that used 16kB for stack.

Author: sg-

features/FEATURE_COMMON_PAL/nanostack-hal-mbed-cmsis-rtos/arm_hal_timer.cpp

@@ -12,7 +12,7 @@
 #include <mbed_assert.h>
 
 static osThreadId_t timer_thread_id;
-static uint64_t timer_thread_stk[2048];
+static uint64_t timer_thread_stk[2048/sizeof(uint64_t)];
 static osRtxThread_t timer_thread_tcb;
 
 static Timer timer;